1. Field of the Invention
This invention relates to fuel cells, and more particularly to electrolyte volume control within a fuel cell.
2. Description of the Prior Art
In a fuel cell, electrolyte is disposed between a pair of spaced apart electrodes. The electrodes often comprise a substrate and a catalyst; the substrate is provided simply to carry the catalyst and must be designed such that during operation the catalyst is in continuous contact with the electrolyte. The electrode must also be constructed to permit the reactant, such as gaseous hydrogen, to enter the substrate and also contact the catalyst. In the prior art it is generally considered that a three-phase interface is formed between the reactant gas, the catalyst, and the dectrolyte, at which place the electrochemical reaction occurs. Many early electrodes, such as those used in the cells described in U.S. Pat. Nos. 2,969,315 and 2,928,783 used porous nickel electrodes wherein the catalyst was distributed uniformly throughout the thickness of the entire electrode. These early cells incorporated a circulating electrolyte so that the water could be either added or removed external of the cell, thereby maintaining a relatively constant volume of electrolyte within the cell. In any event, small changes in electrolyte volume simply changed the location of the three-phase interface within the electrode substrate.
Later cells went to a non-circulating or trapped electrolyte disposed in a matrix sandwiched between the electrodes. In these cells water produced during operation is removed by evaporating it into one of the reactant gas streams. In order to reach the reactant gas stream water vapor must be able to pass through the electrode, yet one could not permit the electrode to completely fill with liquid since this might prevent the reactant gas from entering the electrode to react with the electrolyte at the catalyst sites. Efforts to avoid this type of problem resulted in the development of biporous electrodes. One such biporous electrode is described in U.S. Pat. No. 3,077,508 beginning at line 2 of column 4. As described therein, the biporous structure generally includes a large pore layer on the gas contacting side and a small or fine pore layer on the electrolyte contacting side. The fine pore layer would necessarily be activated with a catalyst. This might also be true of the large pore layer, although it is not a requirement. The high capillary action in the fine pore layer strongly held the electrolyte, while the large pore layer would remain relatively free from electrolyte and would therefore always permit the reactant gas to enter the electrode substrate. The electrochemical reaction took place at approximately the boundary between the large and small pore layers wherein a three-phase interface exists. However, the small pore layers of these early cells were generally very thin such that other provisions were required for electrolyte volume changes.
In an electrode having a catalyst uniformly distributed throughout the substrate it does not matter if, for example, the electrolyte fills half or three-quarters of the electrode thickness since there is always catalyst at the boundary between the electrolyte and reactant gas. Thus, it is only necessary that the reactant gas be able to penetrate through the portion of the electrode not filled with electrolyte. However, electrochemical activity only occurs at the three-phase interface, and catalyst not disposed at that interface does not react and is virtually being wasted. Further development led to electrodes wherein the catalyst was not dispersed throughout the entire substrate, but was rather applied as a very thin layer to the surface of the substrate adjacent the electrolyte. In that type of electrode it is required that there always be gas passageways extending all the way through the substrate to the catalyst layer. In order to ensure that the reactant gas reaches the catalyst layer, it has always been considered necessary to use a hydrophobic substrate which cannot hold significant electrolyte and therefore cannot block the passage of reactant gas through the substrate to the catalyst layer. This is the most common type of electrode in use today. However, in non-circulating electrolyte type cells, it is still necessary to remove excess water by evaporating it into one of the reactant gas streams and/or to be able to store excess electrolyte volume some place within the cell, particularly at shutdown when the water vapor within the gas streams condenses to a liquid. With hydrophobic substrates the condensed water vapor would increase the volume of the electrolyte and may form a film of liquid on the backside or inside the substrate which acts as a barrier to gas flow through the substrate when the cell is put back into operation.
Solutions to the above discussed problems are shown and described in commonly owned U.S. Pat. Nos. 3,779,811 and 3,905,832. In the former patent a porous electrolyte reservoir plate (ERP) is disposed in the reactant gas passage and is spaced from the electrode. Porous pins provide electrolyte communication between the porous plate and the electrode. The electrolyte volume of the cell is controlled by electrolyte movement through the pins of the porous plate, thereby stabilizing the electrochemical performance of the cell and preventing flooding of the electrode. Note that in the embodiment described therein the electrode comprises a conductive nickel screen embedded in a uniform admixture of platinum plus polytetrafluoroethylene particles thereby making the electrode basically hydrophobic. In the U.S. Pat. No. 3,905,832 hydrophilic material is disposed behind and in contact with a hydrophobic electrode substrate to act as an electrolyte reservoir. Communication between the reservoir material and the electrolyte matrix is provided by, for example, holes through the electrode filled with a hydrophilic material or by leaving discrete portions of the electrode substrate hydrophilic to provide wicking paths between the electrolyte matrix and the reservoir material. In this manner excess electrolyte has a place to go without significantly affecting the flow of gas through the hydrophobic areas of the substrate.
While the inventions described in the foregoing two patents work well, they have certain drawbacks. One drawback is increased cell thickness. Another is the increase in IR losses due to either reduced contact between the electrode and separator plate or by the addition of additional material through which the electric current must pass. Increased cost is another problem; this is not only due to the cost of the reservoir layer or material itself, but may also include increased electrode fabrication costs, such as would be required with the invention described in the U.S. Pat. No. 3,905,832.